The present invention relates to the preparation of a new class of photographic stabilizers and their use in photographic silver halide emulsions and materials.
As known to those skilled in the art an antifoggant or fog-inhibiting compound reduces the fog level of a developed freshly coated photographic emulsion by decreasing the rate of development of fog centers relative to the rate of development of latent image; on the other hand stabilizers inhibit fog increase during longer periods of storage of the coated photographic material. Since many classes of useful compounds to some degree exhibit both properties the term "stabilizer" in the following text will refer to compounds eventually exhibiting both functions.
Many known compounds can be added as fog-inhibiting agent or stabilizer to the silver halide emulsion. Suitable examples are i.a. the heterocyclic nitrogen-containing compounds such as benzothiazolium salts, nitroimidazoles, nitrobenzimidazoles, chlorobenzimidazoles, bromobenzimidazoles, mercaptothiazoles, mercaptobenzothiazoles, mercaptobenzimidazoles, mercaptothiadiazoles, aminotriazoles, benzotriazoles (preferably 5-methyl-benzotriazole), nitrobenzotriazoles, mercaptotetrazoles, in particular 1-phenyl-5-mercapto-tetrazole, mercaptopyrimidines, mercaptotriazines, benzothiazoline-2-thione, oxazoline-thione, triazaindenes, tetrazaindenes and pentazaindenes, especially those described by Birr in Z. Wiss. Phot. 47 (1952), pages 2-58, triazolopyrimidines such as those described in GB-A 1,203,757, GB-A 1,209,146, JA-Appl. 75-39537, and GB-A 1,500,278, and substituted 7-hydroxy-s-triazolo-[1,5-a]-pyrimidines as described in U.S. Pat. No. 4,727,017, and other compounds such as benzenethiosulphonic acid, benzenethiosulphinic acid, benzenethiosulphonic acid amide. Other compounds that can be used as fog-inhibiting compounds are metal salts such as e.g. mercury or cadmium salts. Surveys of these and other useful stabilizers are given in Research Disclosure No. 18431 (Aug. 1979) chapter II, and Research Disclosure 307105 (Nov. 1989) chapter VI. The stabilizer most widely used in photographic materials is 7-hydroxy-5-methyl-s-triazolo-[1,5-a]-pyrimidine to which will be referred hereafter as Reference Compound R-1.
The fog-inhibiting agents or stabilizers can be added to the silver halide emulsion prior to, during, or after the ripening thereof and mixtures of two or more of these compounds can be used. Instead of being present in the photographic material itself the stabilizer(s) can be added to one or more processing solutions preferably the development bath.
An effective stabilizing agent showing no unwanted side effects should meet several demands. First of all it should be stable in common alkaline processing conditions. It should be odourless, non-toxic and non-corrosive. It should be a a transparent colourless compound itself and form a colourless compound with silver ions.
From uttermost importance, an ideal stabilizer should reduce the fog level of a photographic material, freshly coated or stored, without adversely affecting the sensitivity and/or gradation. Several classes of known stabilizers reduce the practically obtained sensitivity and/or gradation by hampering the developability of latent image containing silver halide emulsion grains. In some cases a partial solution of this problem can be found in the use of a special class of stabilizers disclosed in Research Disclosure No. 29759 (Jan. 1989) which are in fact an ionic combination of a fog-inhibiting moiety and a development activating moiety. Still there is need for a constant search for new classes of photographic stabilizers showing no desensitizing side effect.
This problem of sensitivity and gradation loss is particularely cumbersome in a special class of photograhic materials known as Diffusion Transfer Reversal materials ("DTR materials"). The principles of this silver complex diffusion transfer reversal process have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).
In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver solvent into soluble silver complex compounds which are allowed to diffuse into an image-receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black chemically developed silver image obtained in the exposed areas of the photographic material.
A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background. For example, typical lithographic printing plates are disclosed e.g. in Japanese Examined Patent Publication (Kokoku) 30562/73, Japanese Unexamined Patent Publications (Kokai) Nos. 21602/78, 103104/79, 9750/81 etc.
The DTR-image can be formed in the image-receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image-receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image-receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.
Mono-sheet DTR materials intended to be used as planographic printing plates are spectrally sensitized to match the emission wavelength of low output lasers which are the illuminating units of modern type- and image-setting devices. Red sensitization for exposure by a HeNe laser or near infra-red sensitization for exposure by a semiconductor laser are preferred. However when such spectrally sensitized mono-sheet DTR materials are stabilized with classically used compounds such as Reference Compound R-1 the chemical developability of the exposed silver halide is so strongly hampered that the differentiation between the DTR silver and the black silver background is almost completely lost. In this way no DTR image with a usable sensitivity and gradation can be obtained and the material cannot be transformed by processing into a lithographic printing plate. A partial solution to this problem consists in using a class of stabilizers characterized by the presence of a hydrophilic solubilizing moiety in their chemical formula, e.g. p-carboxyphenyl-mercapto-tetrazole, and 7-sulfo-naphto-(2,3,-d)-oxazoline-2-thion-sodium salt which latter compound will be referred to furtheron as Reference Compound R-2. However when using this kind of stabilizers the recovery of the lost sensitivity in DTR materials is only partial. So it is clear that there is still a need for the development of new classes of stabilizers which show little or no unwanted side effects, especially no reduction of sensitivity and gradation.
It is a purpose of the present invention to provide such a new class of photographic stabilizers.